challenges in designing 5 ghz 802.11 ac wifi power amplifiers

CHALLENGES IN DESIGNING5 GHZ 802.11AC WIFI POWERAMPLIFIERS

Evolution of IEEE 802.11 Standards

Standard YearReleased

TechnologyDetails

Frequency Bandwidth Highestdata rate

802.11n 2009 OFDM(64-QAM),MIMO

2.4 GHz and5 GHz

20 and 40MHz

1×1: 150 Mb/s4×4: 600 Mb/s

802.11ac 2012 OFDM(256-QAM),MIMO,MU_MIMO

5 GHz only 20, 40, and80 MHz,160 MHz(Optional)

1×1: 866.7Mb/s8×8: 6.77 Gb/s(160 MHz BW)

Drivers for Higher WiFi Data Rates

n Wireless display of high-definition(HD) images andvideo.

Linearity Requirement on 802.11ac Transmitter/PA

n 802.11AC VS. 802.11Nu In order to ensure modulation quality, 256-QAM (802.11ac)

requires more stringent EVM requirement than 64-QAM(802.11n) due to higher constellation density [1]


n 802.11AC VS. 802.11N[8]


n 802.11AC VS. 802.11N

u In 802.11n, the maximum allowed EVM of the WiFitransmitter is -28dB or 4%, when using 64 QAM with acoding rate of 5/6. In order to meet this requirement overvarying temperatures and power supply voltage levels, theWiFi PA or FEM is typically required to achieve an EVMbelow -30.5dB or 3%.

u In 802.11ac, the maximum allowed EVM of the WiFitransmitter has been reduced to -32dB or 2.5% while theWiFi PA usually needs to achieve an EVM below -35dBor 1.8%, when being tested with an 80MHz 11ac MCS9signal (i.e., 256-QAM modulation with a coding rate of5/6) [1].


n 802.11AC VS. 802.11N


n 802.11AC VS. 802.11N

u Take RFMD RFFM8505 802.11ac Front End Module (FEM) forexample, it achieves 19dBm power @ 1.8% EVM specin TX Mode [1]


n 802.11AC VS. 802.11Nu11n PA meets 17.5 dBm output power @ 3% EVM, but can

only meet 10dBm output power @ 1.8% EVM(11ac requirement) [1]

u11n PA needs re-optimization to meet 11ac EVM at sameoutput power

u In other words, in terms of EVM, 11ac is the worst case. Ifthe test result can meet 11ac requirement, it should be ableto meet 11n requirement as well.

Design Challenge 1: Very Low EVM Requirement

n Usually 802.11ac PAs need to achieve better than -35dBor 1.8% EVM [1]

n Very stringent requirements for PA AM/AM and AM/PMdistortion:0.3dB Gain Imbalance or 2 Phase Imbalance can cause1.8% EVM

Design Challenge 1: Very Low EVM Requirement

n Due to very low EVM requirement, the transmitteradopting Direct Up-conversion architecture need morecareful design. Otherwise, the EVM performance is stillbad even though the PA has good EVM performance [9]

Design Challenge 2: Dynamic Operation andtransient Behavior

n WiFi networks utilize Time Division Duplexing (TDD) –PA is pulsed on and off during usage (dynamicoperation)

n Dynamic mode has worse linearity performance thanstatic mode, so dynamic operation needs carefuldesign of PA transient/thermal behavior [1]


n Once PA is on, amplitude must be flat during entiretransmission. Otherwise, any rise or droop contributesto AM/AM distortion and degrades EVM [1]


n Besides, the PA with dynamic mode needs more carefuldesign on Vcc ripple and IR drop than static mode,especially when Pout is maximum. Imperfect Vcc leadsto bad EVM performance as well.

Design Challenge 3: Achieve PAE and LinearitySimultaneously

n Simple way to improve linearity (EVM) is to increase Icc;however, not acceptable to customers because of lowerPAE [1]

n Need to achieve PAE & linearity simultaneously:optimize load, interstage match, bias circuits [1]

Design Challenge 4: Wide Operation Bandwidth

n Wider channel bandwidth of 802.11ac (80/160 MHz):bias circuit must have sufficient bandwidth to avoidclipping signal and resulting in distortion [1]

n Very flat gain and very little phase distortion channel toavoid EVM degradation


n Take RFMD RFFM8505 802.11ac FEM for example, FEMachieves ~29 dB Gain at various 802.11ac channels,and gain is very flat up to 19 dBm output power, toavoid EVM degradation [1]


n RF bandwidth from 5170 to 5835 MHz (~15% fractionalBW), it indicates that PA’s on-die match networkshould adopt multi LC section to achieve enough BW[1]


n Multi LC section match network has wider BW indeed


n But, wider BW leads to smaller Q-factor and moreinsertion loss


n According to the following formula, with constanttarget TX power, the larger the PA post-loss is, thelarger the PA output power will be

Target TX Power(dBm) = PA output power(dBm) – PA post-loss(dB)


n The larger the PA output power is , the worse linearityand more current consumption will be [10]


n Also, due to wide RF bandwidth, the off-chip matchnetworks should be fine-tuned to converge to onepoint near 50 ohm in Smith Chart over the whole bandto ensure all the channels have the identicalperformance

Design Challenge 5: High Operation Frequency

n In general, due to skin effect, the higher frequency is,the more insertion loss(IL) will be. Take theWiFi 2.4 GHz / 5 GHz diplexer for example [11] :


n In 2.4 GHz, the IL is less than 0.5 dB. Nevertheless, in5 GHz, the IL is more than 0.5 dB [11] :

n As mentioned above, with constant target TX power,the larger the PA post-loss is, the larger the PA outputpower and worse linearity will be


n For a 0201 Size 6.8 nH inductor, its SRF(Self ResonantFrequency) is about 6 GHz

n Thus, when the operation frequency:u> 6 GHz => capacitance behavioru< 6 GHz => inductance behavioru= 6 GHz => resistance behavior


n For a 0201 Size 1.5 pF capacitor, its SRF(Self ResonantFrequency) is about 6 GHz

n Thus, when the operation frequency:u> 6 GHz => inductance behavioru< 6 GHz => capacitance behavioru= 6 GHz => resistance behavior


n Hence, when fine-tuning the match networks with 0201size components, the inductor value should NOT belarger than 6.8 nH, and the capacitor value should NOTbe larger than 1.5 pF

n Otherwise, the impedance trajectory in Smith Chart willbe unexpected. An inductor behaves like a capacitor,and a capacitor behaves like an inductor


n The higher operation frequency is, the strongerparasitic effect will be. In other words, it is moredifficult for 5 GHz opeartion to fine-tune matchnetworks than 2.4 GHz

n Thus, 5 GHz PA’s load-pull is more sensitive to thetolerance of LC value, PCB line width, and solderingquality of front-end components. This maylead to yield rate issue during mass production

Conclusion

n Challenges in designing 5 GHz 802.11ac WIFI PA iles in :uVery Low EVM RequirementuDynamic Operation and transient BehavioruAchieve PAE and Linearity SimultaneouslyuWide Operation BandwidthuHigh Operation Frequency

Reference

[1] CHALLENGES IN DESIGNING 5 GHZ 802.11AC WIFI POWER AMPLIFIERS, RFMD

[2] WCN3660 EVM Degradation Issue Technical Note, Qualcomm

[3] SE5516A: Dual-Band 802.11a/b/g/n/ac WLAN Front-End Module, SKYWORKS

[4] 802.11ac Technology Introduction White Paper, RHODE & SCHWARZ

[5] WLAN IEEE 802.11ac Wide bandwidth high speed 802.11ac technology and testing,

RHODE & SCHWARZ

[6] ACPF-7024 ISM Bandpass Filter (2401 – 2482 MHz), AVAGO

[7] WCN36x0(A) RF Matching Guidelines, Qualcomm

[8] MCS Index for 802.11n and 802.11ac Chart

[9] Sources of Error in IQ Based RF Signal Generation

[10] Integration Aids 802.11ac Mobile Wi-Fi Front Ends

[11] Mini filters for multiband devices, TDK

challenges in designing 5 ghz 802.11 ac wifi power amplifiers

Engineering